Summary:
The gold deposits of the Tabakoto project are dominantly hosted in the Birimian, turbiditic sedimentary rocks of the Kofi Formation. These rocks have been intruded by multiple generations of dikes and sills ranging from felsic to mafic composition. The entire package of rocks have been deformed and metamorphosed during the Eburnean orogeny. While individual structures associated each of the deformational events can host mineable deposits, multiple generations of structure enhances the probability of mineable ore.
The deposits can be further divided into three broad types as follows:
- Shear Zone hosted (Ségala and Ségala NW);
- Fracture and cross structure hosted (Dar Salam, Tabakoto, Dioulafoundou and Kofi C), and;
- Intrusive hosted (Djambaye II).
SHEAR ZONE HOSTED DEPOSITS
The Ségala and Ségala NW deposits are considered to be part of a structurally controlled alteration and mineralization system that is hosted by the core of an isoclinal anticline whose axial trace trends ESE (approx. 110°) and dips steeply to the south at approximately -80°. The anticline is made up of somewhat deformed and altered meta-sediments (greywacke and argillite) that display variable intensities of alteration of chlorite, carbonate, sericite, sulphide and silica. A series of quartz stringers and veins intrude this package.
BRITTLE FRACTURE ZONE HOSTED DEPOSITS
At the Dar Salam, Tabakoto and Dioulafoundou and Kofi C Deposits, a geological model for gold mineralization proposed by Nielsen (2004) favours the emplacement of gold-bearing mineralization along structures developed during north-northeast directed isoclinal folding. Mineralization is associated with pervasive silicification and sericite + quartz + Fe-carbonate alteration within and adjacent to dikes and along intersecting north-east, north-west and easterly trending structures. Gold is preferentially hosted within silicified felsic to intermediate dikes and adjacent sedimentary strata along the axial trace of the north-northeast trending Tabakoto anticline.
Veining at Tabakoto displays a protracted history of development with the dominant vein orientations compatible with the development within pre-existing extensional and shear structures formed during NNE directed folding. There is development of moderate to steeply southeast plunging mineralized zones (“ore shoots”) coincident with the intersection of north trending and subsidiary north-east, north-west and easterly trending mineralized corridors. Late reactivation of these structures has facilitated the emplacement of intermediate, mafic and lamprophyre dikes.
INTRUSIVE HOSTED
Most of the gold mineralization at Djambaye II is stockwork-style, hosted within the intrusion, which is different from Tabakoto where the mineralization is predominantly associated with more discrete veining. From previous work (Standing, 2004), including thin section data, the dominant alteration type at Djambaye II was identified as albitization with lesser amounts of silicification (quartz veining) and sulphidation (pyrite and subordinate arsenopyrite).
Mineralization at Tabakoto, Ségala and Kofi is typically associated with disseminated to massive sulphides; pyrite, pyrrhotite, arsenopyrite and in rare instances chalcopyrite and sphalerite. The gold is either associated with quartz veining or moderate to intense silicification and/or albitization. In addition to the silicification, there is evidence of potassic (sericite and biotite), propylitic (chlorite), and carbonate (dolomite to ankerite).
The deposits, identified to date at Tabakoto and Kofi, occur in a number of different environments and exhibit different styles of mineralization.